Field of the Invention
The present invention relates to a pressure-sensitive adhesive layer for optical applications and to a pressure-sensitive adhesive layer-attached optical film having the pressure-sensitive adhesive layer for optical applications.
Examples of the optical film used in the pressure-sensitive adhesive layer-attached optical film of the present invention include a polarizing film, a retardation plate, an optical compensation film, a brightness enhancement film, and a laminate of any combination thereof. The pressure-sensitive adhesive layer-attached optical film of the present invention is suitable for use in optical applications and may be used, for example, in applications for manufacturing image display devices such as liquid crystal display devices, organic electroluminescence (EL) display devices, plasma display panels (PDPs), and electronic paper, and input devices such as touch panels.
Description of the Related Art
The image-forming system of liquid crystal displays or the like requires polarizing elements to be placed on both sides of a liquid crystal cell, and generally polarizing films are bonded thereto. When the polarizing films are bonded to a liquid crystal cell, pressure-sensitive adhesives are generally used. Bonding between an optical film and a liquid crystal cell or between polarizing films is generally performed with a pressure-sensitive adhesive in order to reduce optical loss. In such a case, a pressure-sensitive adhesive layer-attached polarizing film including an optical film and a pressure-sensitive adhesive layer previously formed on one side of the polarizing film is generally used, because it has some advantages such as no need for a drying process to fix the optical film. In general, a release film is attached to the pressure-sensitive adhesive layer of the pressure-sensitive adhesive layer-attached polarizing film.
During the manufacture of a liquid crystal display, the pressure-sensitive adhesive layer-attached optical film is bonded to a liquid crystal cell. In this process, static electricity is generated when the release film is peeled off from the pressure-sensitive adhesive layer of the pressure-sensitive adhesive layer-attached polarizing film. The static electricity generated in this manner may affect the orientation of the liquid crystal in the liquid crystal display to cause a failure. The static electricity may also cause display unevenness when the liquid crystal display operates. For example, the static generation can be suppressed when an antistatic layer is formed on the outer surface of the optical film, but its effect is not high, and there is a problem in which static generation cannot be fundamentally prevented. To suppress static generation in a fundamental position, therefore, the pressure-sensitive adhesive layer is required to have an antistatic function. Concerning means for providing an antistatic function to a pressure-sensitive adhesive layer, for example, it is proposed that an ionic compound should be added to a pressure-sensitive adhesive used to form a pressure-sensitive adhesive layer (Patent Documents 1 and 2).
Patent Document 1 discloses that an ionic solid including an imidazolium cation and an inorganic anion is added to an acryl-based pressure-sensitive adhesive for use on polarizing films. Patent Document 2 discloses that an organic molten salt, such as an onium salt, which is in a liquid state at room temperature and includes a quaternary nitrogen atom-containing cation of 6 to 50 carbon atoms and a fluorine atom-containing anion, is added to an acryl-based pressure-sensitive adhesive for use on polarizing films. There is also proposed a method in which a conductive polymer such as polythiophene is used as a binder to form an antistatic layer between a polarizing film and a pressure-sensitive adhesive layer (Patent Document 3). The pressure-sensitive adhesive layer-attached polarizing film also needs to be durable in the state of adhesion.